An X-ray diagnostic apparatus is an apparatus which irradiates an object with X-rays from an X-ray source, captures the X-rays transmitted through the patient by using an X-ray detector, and generates an X-ray fluoroscopic image as a shade image proportional to the transit dose. An operator such as a doctor or an examination technician diagnoses an object by observing the X-ray fluoroscopic image generated by the X-ray diagnostic apparatus.
Recently, there has been developed a technique (to be also referred to as LCI) of acquiring the projection data of an object from many directions while rotating the X-ray source and the X-ray detector around the object and obtaining a tomographic image of the object by reconstructing the acquired projection data in the same manner as an X-ray computed tomography apparatus. LCI is used for the visualization and the like of an object exhibiting a relatively small density difference among CT values, such as an organ.
In general, LCI is designed to acquire projection data at a predetermined rate on the orbit of an X-ray source and X-ray detector while keeping X-ray irradiation conditions and the rotational speed of the X-ray source and X-ray detector constant.
Since a sectional shape of a human body is generally elliptic, the body thickness changes depending on the projection direction. Therefore, the body thickness increases in the longitudinal direction of the human body, and the dose of incident X-rays on the X-ray detector becomes lower than that in the widthwise direction. Likewise, even at a region, for example, a shoulder region, in which bones extend together in a specific direction, when the projection direction coincides with the above specific direction, the dose of incident X-rays on the X-ray detector decreases. When the dose of incident X-rays on the X-ray detector becomes lower than a predetermined level, circuit noise in the X-ray detector or X-ray photon quantization noise becomes conspicuous, resulting in a decrease in S/N ratio. As a consequence, streak artifacts appear in the reconstructed image in LCI.
In order to obtain a favorable tomographic image of an object by the LCI function, it is necessary to prevent the occurrence of or reduce artifacts like those described above due to the body thickness change of an object and the like.